With miniaturization and high integration of a semiconductor integrated circuit, improvement of resolution is demanded in a semiconductor exposure apparatus (hereinafter, referred to as an “exposure apparatus”). Accordingly, shortening of a wavelength of light emitted from a light source for exposure is proceeding. In recent years, a gas laser apparatus is used for the light source for exposure in place of an existing mercury lamp. Currently, as a gas laser apparatus for exposure, a KrF excimer laser apparatus that emits deep ultraviolet rays of a wavelength of 248 nm and an ArF excimer laser apparatus that emits vacuum ultraviolet rays of a wavelength of 193 nm are used.
As a next-generation exposure technology, liquid immersion exposure is also performed in which liquid is filled between an exposure lens on an exposure apparatus side and a wafer and a refractive index is changed to shorten an apparent wavelength of the light source for exposure. In the liquid immersion exposure using the ArF excimer laser apparatus as the light source for exposure, vacuum ultraviolet rays of a wavelength equivalent to a wavelength of 134 nm in water is applied to the wafer. This technology is called ArF liquid immersion exposure (or ArF liquid immersion lithography).
Since a spectrum line width of natural oscillation of the KrF and ArF excimer laser apparatuses is wide, about 350 pm to about 400 pm, color aberration occurs and resolution is decreased when these projection lenses are used. Therefore, it is necessary to narrow the spectrum line width of a laser beam emitted from the gas laser apparatus until the color aberration becomes ignorable. Accordingly, a line narrowing module including a line narrowing device (such as an etalon and a grating) is provided in a laser resonator of the gas laser apparatus, which achieves narrowing of the spectrum line width. The laser apparatus narrowed in the spectrum line width in this way is called a line narrowing laser apparatus.